Mechanical plating



United States Patent 3,328,197 MECHANICAL PLATIN G John G. Simon, St.Paul, Minn., assignor to Minnesota Mining and Manufacturing Company, St.Paul, Minn, a corporation of Delaware No Drawing. Filed Feb. 8, 1965,Ser. No. 431,783 13 Claims. (Cl. 117-109) This application is acontinuation-in-part of my copending application Ser. No. 41,022, filedJuly 6, 1960, which in turn is in the nature of a continuation-in-partof my application Ser. No. 789,544, filed Jan. 28, 1959, both nowabandoned.

This invention relates to promoter chemicals for facilitating themechanical plating of finely divided metal particles onto metallicsubstrates as smooth, dense, adherent, lustrous protective and/ordecorative platings or coatmgs.

The process of mechanical plating has been known for perhaps a quarterof a century, but it has become of increasing significance during thelast decade. The board principles of the process are well known; see,e.g., British Patent No. 534,888, US. Patents Nos. 2,689,808 and Re.23,861, and other patents and publications. The process is typicallycarried out by placing in a tumbling barrel parts to be plated, platingmetals in the form of minute malleable particles, impact media such asglass beads and cullet, and water. A variety of inorganic and organicchemicals may also be added to promote the mechanical plating action. Asthe tumbling barrel is rotated, the plating metal particles are hammeredagainst the surface of the metallic parts to plated, the impact mediaand the parts themselves serving to flatten the metal particles andinsure complete coverage. A properly applied mechanical plating issmooth, uniform, and provides excellent corrosion protection, at thesame time requiring simple and inexpensive equipment, particularly whencontrasted to electroplating apparatus.

In order to simplify the operation as much as possible and permitunskilled personnel to carry out the process, it is desirable to conductmechanical plating as a batch operation. Desirably the parts to beplated, the impact media, the Water, the metal plating particles, andany promoter chemicals are all added at the start of an operation andallowed to remain, without change, until the operation is completed.Prior to my invention the results obained by this process were sometimesexcellent, but were also unpredictable, erratic, and often wasteful ofthe metal plating particles. I believe that the reason for this residesin the fact that a mechanical plating operation, to be carried out mostefficiently, should involve the gradual addition of the materials usedto carry out the mechanical plating process. Unfortunately, as indicatedpreviously, this is difiicult to achieve and undesirable from a point ofview of holding the cost of the operation to a minimum.

In accordance with my invention, I provide, for the first time insofaras I am aware, a convenient, simple, economical, and highly reliablemethod of carrying out mechanical plating operations. My inventionincludes the provision of a solid bar, or cake, containing apredetermined desirable combination of mechanical plating promoterchemicals. This bar, which is added at the start of a mechanical platingoperation, is gradually disintegrated by mechanical and chemical forcesas the oper ation proceeds, resulting in a gradual release of thepromoter chemical and tending to maintain a constant optimumconcentration of the promoter ingredients.

As ingredients for the mechanical plating promoter bar my invention, Iinclude an acidic first material (or one which hydrolyzes to yieldhydrogen ions), a second material which prevents the prematureagglomeration of ice metal particles in aqueous suspension, and,optionally, such additives as fillers, metal salts, absorbents,adsorbcuts, or chelating agents, or mold release agents to facilitatemanufacture of the plating promoter chemical bars. (Although someagglomeration is essential, it is considered premature if it proceeds sorapidly that plating of large clusters of metal particles takes place,thereby imparting a rough nodular appearance to some areas while leavingother areas almost totally unplated.) When the aforementionedingredients are molded under pressure, with or without the addition ofheat, as desired, the dispersant typically functions as a binder to holdthe entire bar together. If none of the ingredients just named ascomponents of the plating bar is capable of providing a binding action,I add a binder material separately. Ordinarily, however, this is notnecessary.

A wide variety of acid, or acid-engendering materials, may be used inthe practice of my invention, provided the dissociation constant is atleast equal to about 10*. If the dissociation constant exceeds about 10-it will be necessary to add a buffering or protective material to avoidhaving the acid and the plating metal particles consume each otherbefore adequate plating action can take place. The function of theacidic material in this composition is to clean the surface of themetallic substrate to be plated, generally by the removing of oxidescale or other material which would prevent intimate contact between theplating metal particles and substrate, a concept which is well known.This very necessary cleaning action tends to increase the likelihood ofagglomeration. Among the many acids which may be used with effectivenessare the following: acetic, adipic, l-ascorbic, benzoic, butyric, citric,crotonic, diglycolic, formic, gallic, gluconic, hydrochloric, lactic,maleic, malic, phthalic, propionic, succinic, sulfanilic, sulfuric,tartaric, etc. The soluble acidic salts of such acids also proveelfective in this operation. Acidic or acid-engendering materialsinclude aluminum chloride, aluminum sulfate, ammonium 'bifluoride,sodium bisulfate, zinc chloride, zirconium sulfate, and many others.

At one time workers in the field believed that any organic film-formingagent or detergent enhanced the effectiveness of a mechanical platingoperation, and it appears that this is true to at least some extent. Myco-worker Michael Golben has now determined, however, that the class ofmaterials known as dispersants are unexpectedly superior. Although theexact mechanism is not known with certainty, it appears that dispersantsfunction by preventing premature agglomeration of metal platingparticles and the resultant uneven plating. To a considerable extent theplating promoter bar of my invention accomplishes this same end resultmechanically; by insuring a gradual release of acid, the pH of thesolution is maintained high enough to retard reaction between the acidand the metal particles. Accordingly, then, although the plating bar ofmy invention is particularly eifective with those materials which aregood dispersants, it also proves elfective with those materials whichare, e.g., film-forming agents, and have other contributions to make tothe plating process.

Among the materials which can be used effectively are the following: gumcolloids such as gelatin, alginates and agar and tree exudations such asgum acacia, gum karaya, gum tragacanth, and the like, as set forth inUS. Patent 2,689,808; fatty acids or amines, as set forth in US. Patent3,132,043; substituted primary, secondary, or tertiary amines or amides,as set forth in US. Patent 2,640,002; polyoxyethylene adducts of variousamides as set forth in Dutch Patent 98,545; higher alkyl trimethylammonium salts, as set forth in US Patent 2,999,767; long chain tertiaryamines solubilized with ethylene oxide, as set forth in Us. Patent3,032,127; amphoteric proteinaceous l compounds such as gelatine glue;certain metal soaps; polyoxypropylene glycol adducts of higher fattyacid amides; and numerous others heretofore known. I have also foundthat certain discoveries of Michael Golben, my associate, are extremelyeffective. Among these are polyoxyethylene glycol-terminated nonionicsurfactants having a polyoxypropylene-terminated nucleus; anionicsulphonated alkyl aryl salts; compounds containing hydrophilicheterocyclic and hydrophobic alkyl groups.

I have also found that certain high molecular weight polyoxyethyleneglycols are excellent dispersants for making the promoter chemical barof my invention. Although others have suggested the use of low molecularweight glycols in plating operations, I find such materials generallyineffective. Further, a high molecular weight alone is not sufficient toinsure effectiveness, still other criteria proving critical. I find thatthe effective polyoxyethylene glycols are those which have a molecularweight of at least 12,000 and which display a cloud point in 1% solutionbelow 100 C. Speaking in general terms, a branched chain polyoxyethyleneglycol compound need not be of as high a molecular weight as a straightchain polyoxyethylene glycol; Thus, a branched chain polyoxyethyleneglycol may be made by polymerizing three or four straight chainpolyoxyethylene glycol polymers, each having a molecular weight of6,000, on a polyfunctional nucleus such as a diepoxide; the resultantmaterial has a molecular weight in the range of 18,000 to 24,000 andfunctions very Well. On the other hand, a straight chain glycol of thesame molecular Weight does not prove effective; however, a straightchain glycol having a molecular weight of 50,000 works verysatisfactorily. The exact explanation for this is somewhat obscure, butI believe, without being bound thereby, that the effectiveness of theglycol in dispersing the metal plating particles is dependent directlyon the degree to which the glycol polymer interferes with the prematureagglomeration of the plating metal particles.

Regardless of the dispersant used, it is necessary that it be at leastvery slightly soluble as that term is defined in Hackhs ChemicalDictionary, i.e., that one part of dispersant dissolves in from1,000l0,000 parts of water or less, in ten minutes. The amount ofdispersant which is effective in combination with the acidic cleaningagent has been found to be as low as the weight of the active hydrogenion available from the acidic material. The exact amount of dispersantused dependsupon the relative molecular weights of the acidic materialand dispersant, the strength of the acidic material, the effective nessof the dispersant, and the particular system involved. Where thedispersant is a polyoxyethylene glycol of the type previously describedherein, and where the acid is citric acid, the amount of dispersant ispreferably from about 1% to about 4% the weight of the active hydrogenion available from the acid; additional quantities of dispersant serveno very useful purpose and may, in fact, interfere with the platingoperation.

A a guide to securing the proper ratio between dispersant and acidicmaterial I have found it convenient to relate the amount of dispersantto the. amount of available hydrogen ion supplied by the acidicmaterial. As a rule of thumb, at least .05 gram of dispersant, andgenerally at least 1.5 grams, is required for every gram mol ofavailable hydrogen. Available in this context is intended to mean thatthe acidic hydrogen has a dissociation constant of at least 10 It willthus be apparent that some polybasic acids have both available andunavailable hydrogen.

In evaluating the etfectiveness of a given dispersant in producing goodmechanical platings, I have found it convenient to consider sixcriteria-plating efficiency, uniformity, cohesion, coverage, brightness,and smoothness of the mechanical plating. The relative significance ofthese criteria varies with the specific end result sought, but anarbitrary value of 5 is assigned for outstanding performance. Accordingto this somewhat subjective and 4 empirical technique, a perfectmechanical plating would be assigned the value of 30. Generallyspeaking, a value of 22 is considered fair, and 25 or more is consideredvery good.

As a guide to interpreting the criteria specified in the precedingparagraph, I offer the following brief statement of qualities whichwould receive a 5 rating. Plating efficiency-at least of plating metalparticles are utilized. Uniformitythickness variation of no more than*-10% over the plated object. Cohesionplating is not removed from thesubstrate when scored with a razor blade, and tested by applying a stripof pressure-sensitive adhesive tape and quickly removing it.Coverage-none of the uncoated surface shows through. Brightnesssurfaceclosely resembles polished zinc. Smoothness-surface is regular, with nosigns of nodularity.

A typical bar may be made in accordance with my invention by comminutingall solid ingredients, uniformly blending them in, e.g., a twin-shellmixer, and pressing into a desired shape at, e.g., 6,000 p.s.i. BarsWeighing approximately 4 ounces each have proved particularly suitablein the practice of this invention, since no more than ten such barsusually need be added to even the largest mechanical plating, and onebar can be used effectively in the smallest equipment commerciallyavailable.

The following examples illustrate the use of various compositions ofpromoter chemical materials in the practice of my invention. Theseexamples are intended to be illustrative only, and numerous equivalentswill suggest themselves to the man ordinarily skilled in the art. Allparts are by weight unless otherwise noted.

Example 1 1000 grams of shingle brackets were placed in a 1.2 gallon,hexagonal, plastisol-lined steel tumbling mill. The brackets had beenpreviously cleaned by soaking in alkaline solution, pickled in 6% HClsolution and copperflashed by immersion for 15 seconds in a solution of0.24 pound per gallon of Cuprodine salt and 113 milliliters per gallonof sulfuric acid. The mill was then charged as follows:

(a) Impact media: Grams 5 to 6 mesh spherical glass beads 2100 At theend of 60 minutes rotation, a bright, lustrous, uniform 0.0003 inchthick plate had been deposited about the entire surface of the part. Theappearance of the coat approached the smoothness and luster obtainedthrough electroplating; the grain of the coating was not significantlypebbled or rough as is characteristic of many prior art mechanicallyapplied platings. Adherence of the plate was excellent.

In the foregoing example the diammonium citrate serves as a buffer,raising the initial pH of the solution, establishing a more uniform rateof mechanical plating, and facilitating the plating of complex shapes.Where thicker coatings or faster plating action may be desired, theratio of citric acid to diammonium citrate may be increased or, ifdesired, the diammonium citrate may be eliminated altogether. TheCarbowax 20,000 is formed by polymerizing at least three polyoxyethyleneglycol chains having an average molecular weight of 6,000 on a diepoxidenucleus.

Example 2 1000 grams of parachute harness fittings were placed into a1.2 gallon, hexagonal, plastisol-lined steel tumbling mill. The fittingshad been previously cleaned, pickled and copper-flashed as described inExample 1. The mill was then charged as follows:

Grams (a) Impact media:

2 to 4 mesh spherical glass beads 2100 20 to 50 mesh spherical glassbeads 2100 (b) Metal:

Cadmium alloy (60% Cd, 30% Sn, 20% Pb) powder which passes through a 325US.

Tyler sieve (c) Water: at 50 F. to cover charge. (d) Metal plating bar:

Citric acid 5 Diammonium citrate 5 Carbowax 20,000 0.4 Stannous sulfate02 Lead Sulfate 0.2 Stearic acid 0.2

In the metal plating bar composition of this example, combination ofstannous sulfate and lead sulfate seems to be particularly effective inthe plating of cadmium, tin, or lead alloys. The stearic acid functionsas a mold release agent during the formation of the plating bars.

Example 3 4800 grams of clean assorted sheet metal screws were given aflash coating of copper by immersion plating in an acid-copperingsolution. The parts were then rinsed and placed in a 1.2-gallon,hexagonal, plastisoldined steel tumbling mill. The mill was then chargedas follows:

(a) Impact media:

12 to 14 mesh spherical glass beads pints 4 The lid was closed and themill rotated for minutes, after which an additional 12.5 grams ofcadmium powder and 37.5 grams of zinc powder were added. After anadditional 20 minutes of rotation, the parts were separated from themedia by screening and subjected to a 30-second treatment withiridescent chromate solution. These parts showed excellent resistance toboth white and red corrosion in 5% salt spray.

Example 4 56 pounds of brass terminals were cleaned by tumbling them for15 minutes in an acid cleaning solution maintained at 160 F. There werethen given a flash coating of tin and placed in an open-facedvibratory-rotating barrel, together with 90 pounds of spherical glassimpact media, one pound of tin powder, and a /1 pound promoter chemicalbar containing parts citric acid, 25 parts diammonium citrate, 1 partstannous sulfate, 2 parts Carbowax 20,000, 1 part lead sulfate, and 1part stearic acid. After plating for 40 minutes, the brass terminals hadbeen provided with a bright tin coating approximately .3 mil thick andcovering the surface uniformly.

Example 5 25 pounds of deburred, barb shank nails were cleaned in acidcleaner and immersion coppered. They were then placed in avibratory-rotary type plating barrel, to which were added 25 pounds ofspherical glass impact media, sufiicient water to cover the charge, 624grams of zinc powder, and two 4-ounce promoter chemical bars. Eachpromoter chemical bar contained parts of citric acid, ten partsdiammonium citrate, 2 parts Carbowax 20,000, 2 parts stannous sulfate,and 2 parts stearic acid. The barrel was then agitated for 30 minutes,after which an additional 624 grams of Zinc powder and 1 promoterchemical bar of a somewhat diiferent composition was added. This barcontained 75 parts citric acid, 25 parts diammonium citrate, 10 partsstannous sulfate, 2 parts Carbowax 20,000, and 2 parts stearic acid.After an additional 30 minutes of agitation, the parts were found tohave a coating of excellent brightness and a thickness between 1 and 2.3mils.

Example 6 700 pounds of inch x 4 /4 inch bolts were cleaned andfiash-coppered in an open-faced barrel, after which they weretransferred to an octagonal horizontal barrel containing 900 pounds ofspherical glass impact media. Warm water was added to above the level ofthe mixture at a temperature such that the total charge was warmed to 81F. To the barrel contents were now added 3 /2 lbs. of zinc, 1 /2 lbs. (6bars) of promoter chemical composition made up of 54 parts citric acid,54 parts diammonium citrate, 1 part stannous sulfate, 3 parts gumacacia, and 2 parts stearic acid. The barrel was then rotated at 10r.p.m. for 30 minutes. The resultant coating had good brightness,adhesion, coverage, and uniform plating between parts.

Example 7 To a 1.2-gallon plastisol-lined hexagonal barrel was added1,000 grams of %-inch mild steel washers which had been cleaned in anacid cleaning solution and flash coated by immersion in an acidcoppering solution for one minute. To the barrel was then addedspherical glass impact media having the following composition: 1625grams 46 mesh, 845 grams 12-14 mesh, 359 grams of 90-100 mesh. Anll-gram promoter chemical pellet, formed at 80,000 p.s.i., was thenadded to the contents of the barrel. The pellet was composed of 5 gramscitric acid, 5 grams diammonium citate, 0.2 gram stearic acid, and 0.8gram of a mixture of quaternary aliphatic ammonium salts availablecommercially as Arquad S-2C. Next, 20 grams of zinc dust having anaverage particle size of 3 microns was added, after which plating wascarried out at 54 r.p.m. for one hour. The quality index, referred tohereabove, was 27.

Example 8 The process of Example 7 was repeated except that the 0.8 gramof Arquad S2C was replaced with 0.4 gram of Polyglycol E50,000, astraight chain polyoxyethylene glycol having a molecular weight ofapproximately 50,000 available from the Dow Chemical Company. Thequality index of the resultant product was 25. Other straight chainglycols, e.g., a similar product having a molecular weight of only20,000, proved quite ineffective. It is noted that the cloud point ofPolyglycol E50,000, used in this example, is approximately 91 C., whilestraight chain polyoxyethylene glycols having a molecular weight of only20,000 do not display a cloud point. Although I am uncertain why thiscriterion should be significant, I have found that it is extremelyuseful in determining those polyoxyethylene glycols which are effectivefor my pur poses.

Example 9 The process of Example 8 was repeated, except that the amountof Polyglycol E50,000 was reduced to 0.2 gram. Results were essentiallythe same as in Example 8.

Example 10 The process of Example 7 was repeated, except that the 0.8gram of Arquad S-2C was replaced with 0.7 gram of a polyoxyethyleneglycol adduct of oleic amide, available commercially as Nopalcol AO-43from the Nopco Chemical Company. Quality index of the resultant platedproduct was 23.

Example 11 The process of Example 7 was repeated except that 0.4 gram ofSwift and Company Technical Protein Colloid 2185 was employed as adispersant. Quality index of the resultant product was 25. Substantiallyidentical results were obtained when Technical Protein Colloids No. 69and 70 were employed. Likewise, it was found that this hide glue ofvarious viscosities, as well as gelatin, could be substituted withsubstantially the same effectiveness. Surprisingly, it was found thatthe quantity required to produce excellent results was considerably lessthan for any other dispersants which I have tried. For example, onlyabout .05 gram of dispersant is required for each gram mol of availablehydrogen supplied by the acid.

The rate at which promotor chemical bars made in accordance with myinvention release the components thereof is inversely related to thepressure at which the bars are formed. The relationship is not linear,however, and it depends to a considerable degree on the physical natureof the bar components.

The metal plating bar above described may also contain minute platingmetal particles for controlled release into the charge. These metalparticles can be admixed with the other ingredients. To avoidcontamination, oxidation or other types of reaction, the plating metalparticles may receive a protective coating, such as a plastic orresinous coating, abradable by agitation and impact, or they can beincorporated into the bar in the form of distintegrable plastic pelletsor capsules, which capsules form a barrier between the metal and theother ingredients.

Metal plating bars of controlled disintegration containing other binderscan be made, e.g., by mixing and compacting both liquid and solidingredients. In such case it is desirable to add an adsorbent orabsorbent powder, e.g., a hydrophilic, argillaceous powder likeattapulgite clay, diatomaceous earth, fullers earth, and the like.

Other variations will readily occur to the man skilled in the art.

What I claim is:

1. As a means to promote the mechanical plating, in an aqueous medium,of metal powder particles onto a substrate to be plated therewith, awater-disintegrable solid bar capable of gradually releasing platingpromoter chemicals to the aqueous medium over a period of time duringthe plating operation to enhance and facilitate uniformity of plating,said solid bar comprising an acidic first material having a dissociationconstant of at least 10" and a second at least very slightly solublematerial capable of slowing up or preventing the premature agglomerationof metal powder particles in aqueous suspension, said second materialbeing present in an amount equal to at least about .05 gram per gram molof available hydrogen supplied by said acidic material, said barincluding ingredients which impart mechanical stability by binding thebar together, said bar further containing a buffering or protectivematerial where the dissociation constant of said acidic first materialexceeds 10* 2. As a means to promote the mechanical plating, in anaqueous medium, of metal powder particles onto a substrate to be platedtherewith, a water-disintegrable solid bar capable of graduallyreleasing plating promoter chemicals to the aqueous medium over a periodof time during the plating operation to enhance and facilitateuniformity of plating, said solid bar comprising an acidic materialhaving a dissociation constant of at least 10- and a solid, at leastvery slightly soluble, polyoxyethylene glycol having a molecular weightof at least 12,000 and a cloud point in 1% solution below C., saidpolyoxyethylene glycol functioning as a dispersant and further servingto impart stability to said bar by binding the ingredients thereoftogether, said bar further containing a bufi'en'ng or protectivematerial where the dissociation constant of said acidic material exceeds10 3. The product of claim 2 in which the polyoxyethylene glycol is astraight chain glycol having a molecular weight of about 50,000.

4. The product of claim 2 in which the polyoxyethylene glycol is abranched chain glycol having a molecular weight of at least about20,000.

5. The product of claim 2 in which at least about 1.5 grams ofpolyoxyethylene glycol is present for every gram mol of availablehydrogen provided by said acidic material.

6. As a means to promote the mechanical plating, in an aqueous medium,of metal powder particles onto a substrate to be plated therewith, awater-disintegrable solid bar capable of gradually releasing platingpromoter chemicals to the aqueous medium over a period of time duringthe plating operation to enhance and facilitate uniformity of plating,said solid bar comprising an acidic material having a dissociationconstant of at least 10-", a small amount of a water-soluble tin salt,and an at least very slightly soluble dispersant for metal particles inagitated acid solution capable of slowing up or prevent-. ing thepremature agglomeration of metal powder particles, said dispersant beingpresent in an amount equal to at least about .05 gram per gram mol ofavailable hydrogen supplied by said acidic material, said bar includingingredients which impart mechanical stability by binding the bartogether, said bar further containing a buffering or protective materialwhere the dissociation constant of said acidic material exceeds 10 7. Asa means to promote the mechanical plating, in an aqueous medium, ofmetal powder particles onto a substrate to be plated therewith, awater-disintegrable solid bar capable of gradually releasing platingpromoter chemicals to the aqueous medium over a period of time duringthe plating operation to enhance and facilitate uniformity of plating,said solid bar comprising an acidic material having a dissociationconstant of at least '10- and an at least very slightly solubledispersant for metal particles in agitated acid solution materialcapable of slowing up or preventing the premature agglomeration of metalpowder particles, said dispersant being present in an amount equal to atleast about .05 gram per gram mol of available hydrogen supplied by saidacidic material, said bar including a chelating agent and ingredientswhich impart mechanical stability by binding the bar together, said barfurther containing a buffering or protective material where thedissociation constant of said acidic material exceeds 10- 8. In a methodof mechanically plating parts which includes the steps of placing saidparts in a plating barrel together with finely divided plating metal,impact media, acid and water, and thereafter agitating the contents ofthe barrel until said parts have been plated with said plating metal,the improvement comprising adding to the contents of said barrel an atleast very slightly soluble nonionic material which consists essentiallyof a polyoxyethylene glycol having a molecular weight of at least 10 and(2) an at least very slightly soluble polyoXyethylene glycol having amolecular weight of at least about 12,000 and a cloud point in 1%solution below 100 C., the ratio of said glycol to said acid being suchthat there is at least 1.5 grams of glycol per gram mol of hydrogenavailable from said acid, said bar further containing a buffering orprotective material where the dissociation constant of said acid exceeds10 10. As a mechanical plating promoter, the combination of (1) citricacid and (2) an at least very slightly soluble polyoxyethylene glycolhaving a molecular weight of at least about 12,000 and a cloud point in1% solution below 100 C., the ratio of said glycol to said acid beingsuch that there is at least 1.5 grams of glycol per gram mol of hydrogenavailable from said acid.

11. As a mechanical plating promoter, the combination of (1) citric acidand (2) an at least very slightly soluble adduct of a short-chainpolyfunctional compound and at least three molecules of polyoxyethyleneglycol, the molecular weight of said adduct being at least about 20,000,the ratio of said glycol to said acid being such that there is at least1.5 grams of glycol per gram mol of hydrogen available from said acid.

12. As a promoter for use in the mechanical plating of cadmium, thecombination of (1) an acid having a dissociation constant of at least 10(2) an at least very slightly soluble polyoxyethylene glycol having amolecular weight of at least about 12,000 and a cloud point in 1%solution below 100 C., the ratio of said glycol to said acid being suchthat there is at least 1.5 grams of glycol per gram mol of hydrogenavailable from said acid, (3) a tin salt, (4) a lead salt, and (5) wherethe dissociation constant of said acid exceeds a buffering or protectivematerial.

13. As a means to promote the mechanical plating of metal powderparticles onto a substrate to be plated therewith, a water-disintegrablesolid bar capable of gradually releasing plating promoter chamicals tothe aqueous medium over a period of time during the plating operation toenhance and facilitate uniformity of plating, said solid bar comprisingan acidic material having a dissociation constant of at least 10 a smallamount of a water-soluble tin salt, a small amount of lead salt, and anat least very slightly soluble dispersant for metal particles inagitated acid solution capable of slowing up or preventing the prematureagglomeration of metal powder particles, said dispersant being presentin an amount equal to at least about .05 gram per gram mol of availablehydrogen supplied by said acidic material, said bar includingingredients which impart mechanical stability by binding the bartogether, said bar further containing a buffering or protective materialwhere the dissociation constant of said acidic material exceeds 10References Cited UNITED STATES PATENTS 2,434,855 1/ 1948 Kosterlitzky1061 X 2,519,672 8/1950 Lawless 1061 2,640,002 5/1953 Clayton '1171092,689,808 9/1954 Clayton 117109 X 2,793,965 5/1957 Myers et a1. 1061 X2,940,867 6/1960 Streicher 117130 X 3,023,127 2/1962 Clayton 117131 XOTHER REFERENCES Carbowax Compounds and Polyethylene Glycols, by Carbideand Carbon Chemicals Corp., New York, N.Y., June 30, 1946.

Hill, F. N., et al.: Industrial and Engineering Chemistry, HighMolecular Weight Polymers of Ethylene Oxide, Hill, F. N., et al.,Detergent Digest, vol. 50, No. 1, January 1958, p. 5.

Carbowax, Polyethylene Glycols, Union Carbide Chemicals Co., New York,N.Y., 1960 pamphlet, pp. 1, 9 and 22.

ALFRED L. LEAVITT, Primary Examiner.

JOSEPH B. SPENCER, RALPH S. KENDALL,

Examiners. J. P. MCINTOSH, Assistant Examiner.

8. IN A METHOD OF MECHANICALLY PLATING PARTS WHICH INCLUDES THE STEPS OFJPLACING SAID PARTS IN A PLATING BARREL TOGETHER WITH FINELY DIVIDEDPLATING METAL, IMPACT MEDIA, ACID AND WATER, AND THEREAFTER AGITATINGTHE CONTENTS OF THE BARREL UNTIL SAID PARTS HAVE BEEN PLATED WITH SAIDPLATING METAL, THE IMPROVEMENT COMPRISING ADDING TO THE CONTENTS OF SAIDBARREL AN AT LEAST VERY SLIGHTLY SOLUBLE NONIONIC MATERIAL WHICHCONSISTS ESSENTIALLY OF A POLYOXYETHYLENE GLYCOL HAVING A MOLECULARWEIGHT OF AT LEAST ABOUT 12,000 AND A CLOUD POINT IN 1% SOLUTION BELOW100*C., THE AMOUNT OF SAID MATERIAL ADDED BEING AT LEAST ABOUT 1.5 GRAMSPER GRAM MOL OF HYDROGEN AVAILABLE FROM SAID ACID.